1. Field of the Invention
The present invention relates generally to standards for calibrating ultrasonic test equipment, and more particularly to a substantially solid reference standard having a void of known dimensions contained therein at a known distance from the end of the standard adapted to be engaged by the test equipment.
2. Description of the Prior Art
Ultrasonic test equipment for detecting flaws in metallic and non-metallic parts is widely used for non-destructive inspection of raw and finished materials. Pulsed ultrasonic sound waves generated by a transmitting transducer such as piezoelectric crystal are transmitted into the parts to be inspected, such that any internal defect or flaw, such as a crack, void or other discontinuity in the part, will cause a portion of the sound waves impinging on the defect to be reflected back toward the same or a different receiving transducer, which converts the reflected wave into an electrical signal. The time interval between the transmission of the sound pulses to the body and the reception of the reflected sound pulse from the defect is a measure of the location of the defect. In addition, the magnitude of the reflected pulse is a measure of the size of the defect as viewed in the direction of travel of the sound waves.
Historically, ultrasonic test equipment of this type has been calibrated by using block-like reference standards in which a plurality of holes of various depths and diameters are drilled. However, such standards have not proven entirely satisfactory due to the difficulty in maintaining parallelism between the bottom surface of the hole and the transducer-abutting surface of the standard reference, as well as the surface finish at the bottom surface of the hole, both of which contribute to variances in the reflected sound waves and hence an unreliable reference calibration signal. For example, it has been found that the "flat bottomed" hole in prior art standards is not truly flat, but contains sufficient curvature and irregularities to introduce significant calibration errors in ultrasonic waves reflected from its surface. In addition, since such holes may be of a very small diameter, the drill bit used to produce the hole may "walk", resulting in holes of nonlinear axis or varying cross sectional area. The interior surfaces of such holes produce erratic reflection of ultrasonic energy which reduces calibration accuracy. Furthermore, this structure does not accurately duplicate the real-life flaw environment, wherein a void or other discontinuity is located completely within a surrounding solid body. In addition, prior art reference standards have not been able to simulate foreign particulate inclusions and the like located within the void. Finally, known reference standards used with test equipment to dynamically test rotating bar stock have heretofore been incapable of providing a dynamic calibration signal.